The page description diagram is a design artifact developed by Dan Brown to take design out of wireframes by describing content areas of the page. The approach provides functional specifications and interaction requirements, and lays out blocks of content without limiting the designer's ability to make decisions in the visual design. They are not necessarily meant to replace wireframes, but may be used in conjunction with them
The excerpt below is from Dan Brown's Boxes and Arrows article which describes the use of page description diagrams.
"In a page description diagram, the content areas of the page are described in prose, as in a functional specification. The content area descriptions are arranged on the page in priority order. Typically, I will define the horizontal axis of the diagram as the page priority. Thus, content areas described on the left side of the page are higher priority than those on the right side of the page.
With this approach, the diagram represented the two main issues: priority and content. I found that I could include layouts of individual content areas to show, for example, how the “check flight status” form might look. These mini-layouts helped our client visualize the interactivity, but did not lock the designer into any particular approach. Our conversations with the client focused on the nature of the content and functionality and the relative priority of the page contents."
Source: "Where the Wireframes Are: Special Deliverable #3.", Dan Brown. Boxes and Arrows.